1. Field of the Invention
The present invention relates to a sound synthesizing apparatus and method, adapted to synthesize at a receiving side a wide-band signal from an input narrow-band sound signal or its parameters transmitted by a communications system or a broadcasting system, for example. The present invention also relates to a telephone apparatus adopting the sound synthesizing apparatus and method, and a program service medium by which the sound synthesizing method is served as a program.
2. Description of the Related Art
The sound quality of the conventional wire telephone and radio telephone has not satisfied the telephone users. One of the reasons for such a low sound quality lies in the fact that the frequency band of the current telephony is limited to a range of 300 to 3,400 Hz.
Since the transmission path for use in the telephony is limited by the relevant rules and standards, it is difficult to widen the frequency band. For a higher sound quality in the field of the telephony, various methods have been proposed to predict at the receiving side an out-of-band component of a received sound and generate a wider-band signal.
Typically, there has been proposed a method in which based on the well-known method for linear predictive coding (LPC) analysis and synthesis, used in the sound signal processing, both a linear predictive factor xcex1 acquired from a narrow-band sound signal and a linear prediction residual or an excitation source acquired by quantizing the residual are band-widened and a wide-band sound is synthesized by the LPC from the band-widened linear predictive factor xcex1 and excitation source.
However, since the wide-band sound thus acquired is distorted, the frequency component of the original sound is filtered out of the synthesized wide-band sound and it is added to the original sound.
There has also been proposed an excitation source frequency band widening method in which taking in consideration the fact that an excitation source is a nearly white noise, a zero is inserted between two successive samples to generate an aliasing component and this component is taken as a wide-band excitation source.
When one zero is inserted between two successive samples, for example, the spectrum will appear symmetrical with respect to the Nyquist frequency taken as a line. Therefore, this method will be somehow effective for acquiring a wide-band excitation source from a narrow-band excitation source which is originally a nearly white noise.
On the assumption that the sampling frequency of a narrow-band signal is 8 kHz, that of a wide-band signal is 16 kHz and a narrow-band excitation source is limited to 300 to 3,400 Hz, for example, the wide-band excitation source acquired by the above-mentioned method will be of 300 to 3,400 Hz and 4,600 to 7,700 Hz with a gap between 3,400 and 4,600 Hz. Thus, the frequency band corresponding to this gap will not be generated even by the wide-band LPC synthesis but a wide-band sound not containing a frequency band corresponding to the gap will be generated. Thus, the wide-band sound is not any natural sound.
As in the above, since the excitation source resulted from the LPC synthesis including the band widening, etc. is low in quality, the synthesized signal will also have a low quality.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing a sound synthesizing apparatus and method capable of synthesizing a quality wide-band signal through improvement of the quality of the excitation source.
It is another object of the present invention to provide a telephone apparatus having a receiving means capable of providing a quality wide-band signal by adopting the above sound synthesizing apparatus and method.
It is further object of the present invention to provide a program service medium serving the sound synthesizing method in the form of a program and thus capable of providing a quality wide-band signal inexpensively.
According to the present invention, there is provided a sound synthesizing apparatus for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the apparatus including means for adding a noise signal to the linear prediction residual or excitation source.
According to the present invention, there is also provided a sound synthesizing apparatus for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the apparatus including means for generating a wide-band excitation source from the linear prediction residual or excitation source and means for adding a noise signal to the wide-band excitation source.
According to the present invention, there is also provided a sound synthesizing apparatus for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the apparatus including means for adding a noise signal to the linear prediction residual or excitation source and means for generating a wide-band excitation source from the linear prediction residual or excitation source to which the noise signal has been added by the noise adding means.
According to the present invention, there is also provided a sound synthesizing apparatus for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the apparatus including means for analyzing the narrow-band signal to provide a linear prediction residual signal, means for generating a wide-band residual signal from the linear prediction residual acquired by means of the analyzing means, and means for adding to the wide-band residual signal a noise signal having a signal component whose frequency is not included in the frequency band of the wide-band residual signal generated by the wide-band residual signal generating means.
According to the present invention, there is also provided a sound synthesizing apparatus for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the apparatus including means for analyzing the narrow-band signal to provide a linear prediction residual signal; means for adding to the linear prediction residual signal a noise signal having a signal component whose frequency is not included in the frequency band of the linear prediction residual signal generated by the analyzing means; and means for generating a wide-band residual signal from the linear prediction residual signal to which the noise signal has been added by the noise adding means.
According to the present invention, there is also provided a sound synthesizing method for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the method including a step of adding a noise signal to the linear prediction residual or excitation source.
According to the present invention, there is also provided a sound synthesizing method for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the method including steps of generating a wide-band excitation source from the linear prediction residual or excitation source and adding a noise signal to the wide-band excitation source.
According to the present invention, there is also provided a sound synthesizing method for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the method including steps of the sound synthesizer including means for adding a noise signal to the linear prediction residual or excitation source, and generating a wide-band excitation source from the linear prediction residual or excitation source to which the noise signal has been added at the noise adding step.
According to the present invention, there is also provided a sound synthesizing method for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the method including steps of analyzing the narrow-band signal to provide a linear prediction residual signal, generating a wide-band residual signal from the linear prediction residual acquired at the analyzing step, and adding to the wide-band residual signal a noise signal having a signal component whose frequency is not included in the frequency band of the wide-band residual signal generated by the wide-band residual signal generating means.
According to the present invention, there is also provided a sound synthesizing method for synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the method including steps of analyzing the narrow-band signal to provide a linear prediction residual signal, adding to the linear prediction residual signal a noise signal having a signal component whose frequency is not included in the frequency band of the linear prediction residual signal acquired at the analyzing step, and generating a wide-band residual signal from the linear prediction residual signal to which the noise signal has been added at the noise adding step.
With the sound synthesizing apparatus and method according to the present invention, it is possible to improve the quality of the excitation source and thus provide a quality wide-band signal.
According to the present invention, there is also provided a telephone apparatus including a transmitting means for transmitting parameters of a narrow-band signal encoded by the PSI-CELP or VSELP method as a transmission signal, and a receiving means for adding a noise signal to a linear prediction residual or excitation source included in the parameters and synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis.
According to the present invention, there is also provided a telephone apparatus including a transmitting means for transmitting parameters of a narrow-band signal encoded by the PSI-CELP or VSELP method as a transmission signal, and a receiving means for generating a wide-band excitation source from a linear prediction residual or excitation source included in the parameters, adding a noise signal to the wide-band excitation source and then synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis.
According to the present invention, there is also provided a telephone apparatus including a transmitting means for transmitting parameters of a narrow-band signal encoded by the PSI-CELP or VSELP method as a transmission signal, and a receiving means for adding a noise signal to a linear prediction residual or excitation source included in the parameters, generating a wide-band excitation source from the linear prediction residual or excitation source to which the noise signal has been added, and synthesizing a wide-band signal from a part of an output signal acquired by a filtering synthesis using the wide-band excitation source.
In the telephone apparatus according to the present invention, the receiving means can provide a quality wide-band signal.
According to the present invention, there is provided a program service medium for providing a sound synthesis program for synthesis of a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the program including procedures of generating a wide-band excitation source from the linear prediction residual or excitation source, and adding a noise signal to the wide-band excitation source.
According to the present invention, there is provided a program service medium for providing a sound synthesis program for synthesis of a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the program including procedures of adding a noise signal to the linear prediction residual or excitation source, and generating a wide-band excitation source from the linear prediction residual or excitation source to which the noise signal has been added in the noise adding procedure.
According to the present invention, there is provided a program service medium for providing a sound synthesis program for synthesis of a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the program including procedures of analyzing the narrow-band signal to provide a linear prediction residual signal, generating a wide-band residual signal from the linear prediction residual signal acquired in the analyzing procedure, and adding to the wide-band residual signal a noise signal having a signal component whose frequency is not included in the frequency band of the wide-band residual signal generated in the wide-band residual signal generating procedure.
According to the present invention, there is provided a program service medium for providing a sound synthesis program for synthesis of a wide-band signal from a part of an output signal acquired by a filtering synthesis whose input parameter is a linear prediction residual or excitation source of a narrow-band signal, the program including procedures of analyzing the narrow-band signal to provide a linear prediction residual signal, adding to the residual signal a noise signal having a signal component whose frequency is not included in the frequency band of the linear prediction residual signal acquired in the analyzing procedure, and generating a wide-band residual signal from the linear prediction residual signal to which the noise signal has been added in the noise adding procedure.
The program service medium according to the present invention can provide a quality wide-band signal by serving the sound synthesizing method in the form of a program.
That is, a noise signal is intentionally added to a signal which would originally be an excitation source, in order to improve the quality of a synthesized signal.
More specifically, a noise signal whose gain has been adjusted with the power of a narrow-band excitation source and whose frequency ranges from 3,400 to 4,600 Hz, is generated separately, and added to a wide-band excitation source acquired by zero-filling. A resultant signal is taken as a wide-band excitation source. Alternately, a noise signal of 3,400 to 4,000 Hz is generated separately, added to a narrow-band excitation source, and then filled with zeros. A resulted signal is taken as a wide-band excitation source. Thus, the gap between the frequencies of 3,400 and 4,600 Hz can be eliminated.
In the aforementioned sound synthesizing apparatus and method, a linear predictive factor xcex1 and an excitation source or prediction residual exc are given, and the separately produced noise signal is added to the prediction residual exc. The resultant signal will be referred to as xe2x80x9cexcxe2x80x9d hereinafter. It is supplied to a synthesis filter in which with the linear predictive factor xcex1 taken as its filter factor, it is filtered to provide an output signal.
A filter factor xcex1N used for synthesis of a narrow-band signal has the band thereof widened by any predictive means to provide a wide-band filter factor xcex1W. The excitation source or prediction residual excN is made an aliased signal by zero-filling. The separately produced noise signal is added to the excitation source or prediction residual. The resulted signal will be referred to as xe2x80x9cexcWxe2x80x9d hereinafter. Thereafter, the signal excW is supplied to the synthesis filter having the wide-band filter factor xcex1W, where it is filtered to provide an output signal.
Also, the filter factor xcex1N used for synthesis of a narrow-band signal is band-widened by any predictive means to provide a wide-band filter factor xcex1W. The excitation source or prediction residual excN has the separately produced noise signal added thereto, and further is made an aliased signal by zero-filling. The resulted signal will be referred to as xe2x80x9cexcWxe2x80x9d hereinafter. Thereafter, the signal excW is supplied to the synthesizing filter having the wide-band filter factor xcex1W, where the signal is filtered to provide an output signal.
Also, an input narrow-band signal is subject to a linear predictive analysis or the like to provide a narrow-band factor xcex1N. This narrow-band factor xcex1N is reversely filtered to provide a prediction residual signal excN, and its frequency band is widened by any predictive means to provide a wide-band filter factor xcex1W. The excitation source or prediction residual excN is made an aliased signal by zero-filling and has the separately produced noise signal added thereto. The resulted signal will be referred to as xe2x80x9cexcWxe2x80x9d hereinafter. Thereafter, the signal excW is supplied to the synthesis filter taking the wide-band filter factor xcex1W as its filter factor and in which the signal is filtered to provide an output signal.
Also, a narrow-band signal is subject to a linear predictive analysis or the like to provide a narrow-band factor xcex1N. This narrow-band factor xcex1N is reversely filtered to provide a prediction residual signal excN, and is band-widened by any predictive means to provide a wide-band filter factor xcex1W. The excitation source or prediction residual excN has the separately produced noise signal added thereto and is made a signal which is aliased by zero-filling. The resulted signal will be referred to as xe2x80x9cexcWxe2x80x9d hereinafter. Then, the signal excW is supplied to the synthesizing filter taking the wide-band filter factor xcex1W as its filter factor and in which the signal is filtered to provide an output signal.